Hiya BigH,
There are so many resources out there it's hard to list them all, but if you do a Google search for CSS Tutorials or CSS Courses you should find a lot of them and many of them are pretty good. Mind that you watch the dates on them though so you don't end up learning really old stuff. Anything from 2009 and up and even 2008 should be safe enough to start out with.
I'm thinking though that you're expecting to just put in a CSS file and your site will magically change to the new design, but.... I have bad news for you. To change the design of a site means you pull it apart manually and take the content from the old one and put it into the new template setup. There's really no easy way to do that unless you have a fairly good setup already that just needs a bit of refreshing. If you're looking for a total makeover you're going to have to do a bit of work and get your hands quite dirty.
That's not necessarily a bad thing though. You learn an awful lot by doing that and the forums here are great for helping with specifics.
If you have a link to the current site that always helps people know how to steer you in the right direction too. Good luck on your site updating